Multispherical shapes of vesicles highlight the curvature elasticity of biomembranes

Mode of molecular interaction of triterpenoid saponin ginsenoside Rh2 with membrane lipids in liquid-disordered phases

Ginsenoside Rh2 (Rh2) is a ginseng saponin comprising a triterpene core and one unit of glucose and has attracted much attention due to its diverse biological activities. In the present study, we used small-angle X-ray diffraction, solid-state NMR, fluorescence microscopy, and MD simulations to investigate the molecular interaction of Rh2 with membrane lipids in the liquid-disordered (Ld) phase mainly composed of palmitoyloleoylphosphatidylcholine compared with those in liquid-ordered (Lo) phase mainly composed of sphingomyelin and cholesterol. The electron density profiles determined by X-ray diffraction patterns indicated that Rh2 tends to be present in the shallow interior of the bilayer in the Ld phase, while Rh2 accumulation was significantly smaller in the Lo phase. Order parameters at intermediate depths in the bilayer leaflet obtained from 2H NMR spectra and MD simulations indicated that Rh2 reduces the order of the acyl chains of lipids in the Ld phase. The dihydroxy group and glucose moiety at both ends of the hydrophobic triterpene core of Rh2 cause tilting of the molecular axis relative to the membrane normal, which may enhance membrane permeability by loosening the packing of lipid acyl chains. These features of Rh2 are distinct from steroidal saponins such as digitonin and dioscin, which exert strong membrane-disrupting activity.

Which Coverages of Arc-Shaped Proteins Are Required for Membrane Tubulation?

Arc-shaped BIN/Amphiphysin/Rvs (BAR) domain proteins generate curvature by binding to membranes and induce membrane tubulation at sufficiently large protein coverages. For the amphiphysin N-BAR domain, Le Roux et al., Nat. Commun. 2021, 12, 6550, measured a threshold coverage of 0.44 ± 0.097 for nanotubules emerging from the supported lipid bilayer. In this article, we systematically investigate membrane tubulation induced by arc-shaped protein-like particles with coarse-grained modeling and simulations and determine the threshold coverages at different particle-particle interaction strengths and membrane spontaneous curvatures. In our simulations, the binding of arc-shaped particles induces a membrane shape transition from spherical vesicles to tubules at a particle threshold coverage of about 0.5, which is rather robust to variations of the direct attractive particle interactions or spontaneous membrane curvature in the coarse-grained model. Our study suggests that threshold coverages of around or slightly below 0.5 are a general requirement for membrane tubulation by arc-shaped BAR domain proteins.

Multiscale remodeling of biomembranes and vesicles

Biomembranes and vesicles cover a wide range of length scales. Indeed, small nanovesicles have a diameter of a few tens of nanometers whereas giant vesicles can have diameters up to hundreds of micrometers. The remodeling of giant vesicles on the micron scale can be observed by light microscopy and understood by the theory of curvature elasticity, which represents a top-down approach. The theory predicts the formation of multispherical shapes as recently observed experimentally. On the nanometer scale, much insight has been obtained via coarse-grained molecular dynamics simulations of nanovesicles, which provides a bottom-up approach based on the lipid numbers assembled in the two bilayer leaflets and the resulting leaflet tensions. The remodeling processes discussed here include the shape transformations of vesicles, their morphological responses to the adhesion of condensate droplets, the instabilities of lipid bilayers and nanovesicles, as well as the topological transformations of vesicles by membrane fission and fusion. The latter processes determine the complex topology of the endoplasmic reticulum.

Alkyl anchor-modified artificial viral capsid budding outside-to-inside and inside-to-outside giant vesicles

The budding of human immunodeficiency virus from an infected host cell is induced by the modification of structural proteins bearing long-chain fatty acids, followed by their anchoring to the cell membrane. Although many model budding systems using giant unilamellar vesicles (GUVs) induced by various stimuli have been developed, constructing an artificial viral budding system of GUVs using only synthesized molecules remains challenging. Herein, we report the construction of an artificial viral capsid budding system from a lipid bilayer of GUV. The C-terminus of the β-annulus peptide was modified using an octyl chain as an alkyl anchor via a disulfide bond. The self-assembly of the β-annulus peptide with an octyl chain formed an artificial viral capsid aggregate. The fluorescence imaging and transmission electron microscopy observations revealed that the addition of the tetramethylrhodamine (TMR)-labeled octyl chain-bearing β-annulus peptide to the outer aqueous phase of GUV induced the budding of the capsid-encapsulated daughter vesicle outside-to-inside the mother GUV. Conversely, the encapsulation of the TMR-labeled octyl chain-bearing β-annulus peptide in the inner aqueous phase of GUV induced the budding of the capsid-encapsulated daughter vesicle inside-to-outside the mother GUV. Contrarily, the addition of the TMR-labeled β-annulus peptide to GUV barely induced budding. It was demonstrated that the higher the membrane fluidity of GUV, the more likely budding would be induced by the addition of the alkyl anchor-modified artificial viral capsid. The simple virus-mimicking material developed in this study, which buds off through membrane anchoring, can provide physicochemical insights into the mechanisms of natural viral budding from cells.

Intravesicular Solute Delivery and Surface Area Regulation in Giant Unilamellar Vesicles Driven by Cycles of Osmotic Stresses

Phospholipid bilayers are dynamic cellular components that undergo constant changes in their topology, facilitating a broad diversity of physiological functions including endo- and exocytosis, cell division, and intracellular trafficking. These shape transformations consume energy, supplied invariably by the activity of proteins. Here, we show that cycles of oppositely directed osmotic stresses─unassisted by any protein activity─can induce well-defined remodeling of giant unilamellar vesicles, minimally recapitulating the phenomenologies of surface area homeostasis and macropinocytosis. We find that a stress cycle consisting of deflationary hypertonic stress followed by an inflationary hypotonic one prompts an elaborate sequence of membrane shape changes ultimately transporting molecular cargo from the outside into the intravesicular milieu. The initial osmotic deflation produces microscopic spherical invaginations. During the subsequent inflation, the first subpopulation contributes area to the swelling membrane, thereby providing a means for surface area regulation and tensional homeostasis. The second subpopulation vesiculates into the lumens of the mother vesicles, producing pinocytic vesicles. Remarkably, the gradients of solute concentrations between the GUV and the daughter pinocytic vesicles create cascades of water current, inducing pulsatory transient poration that enable solute exchange between the buds and the GUV interior. This results in an efficient water-flux-mediated delivery of molecular cargo across the membrane boundary. Our findings suggest a primitive physical mechanism for communication and transport across protocellular compartments driven only by osmotic stresses. They also suggest plausible physical routes for intravesicular, and possibly intracellular, delivery of ions, solutes, and molecular cargo stimulated simply by cycles of osmotic currents of water.

Mesoscale simulation of biomembranes with FreeDTS

We present FreeDTS software for performing computational research on biomembranes at the mesoscale. In this software, a membrane is represented by a dynamically triangulated surface equipped with vertex-based inclusions to integrate the effects of integral and peripheral membrane proteins. Several algorithms are included in the software to simulate complex membranes at different conditions such as framed membranes with constant tension, vesicles and high-genus membranes with various fixed volumes or constant pressure differences and applying external forces to membrane regions. Furthermore, the software allows the user to turn off the shape evolution of the membrane and focus solely on the organization of proteins. As a result, we can take realistic membrane shapes obtained from, for example, cryo-electron tomography and backmap them into a finer simulation model. In addition to many biomembrane applications, this software brings us a step closer to simulating realistic biomembranes with molecular resolution. Here we provide several interesting showcases of the power of the software but leave a wide range of potential applications for interested users.

Multispherical shapes of vesicles with intramembrane domains

Phase separation of biomembranes into two fluid phases, a and b, leads to the formation of vesicles with intramembrane a- and b-domains. These vesicles can attain multispherical shapes consisting of several spheres connected by closed membrane necks. Here, we study the morphological complexity of these multispheres using the theory of curvature elasticity. Vesicles with two domains form two-sphere shapes, consisting of one a- and one b-sphere, connected by a closed ab-neck. The necks' effective mean curvature is used to distinguish positive from negative necks. Two-sphere shapes of two-domain vesicles can attain four different morphologies that are governed by two different stability conditions. The closed ab-necks are compressed by constriction forces which induce neck fission and vesicle division for large line tensions and/or large spontaneous curvatures. Multispherical shapes with one ab-neck and additional aa- and bb-necks involve several stability conditions, which act to reduce the stability regimes of the multispheres. Furthermore, vesicles with more than two domains form multispheres with more than one ab-neck. The multispherical shapes described here represent generalized constant-mean-curvature surfaces with up to four constant mean curvatures. These shapes are accessible to experimental studies using available methods for giant vesicles prepared from ternary lipid mixtures.

Pulmonary Surfactant: A Mighty Thin Film

Pulmonary surfactant is a critical component of lung function in healthy individuals. It functions in part by lowering surface tension in the alveoli, thereby allowing for breathing with minimal effort. The prevailing thinking is that low surface tension is attained by a compression-driven squeeze-out of unsaturated phospholipids during exhalation, forming a film enriched in saturated phospholipids that achieves surface tensions close to zero. A thorough review of past and recent literature suggests that the compression-driven squeeze-out mechanism may be erroneous. Here, we posit that a surfactant film enriched in saturated lipids is formed shortly after birth by an adsorption-driven sorting process and that its composition does not change during normal breathing. We provide biophysical evidence for the rapid formation of an enriched film at high surfactant concentrations, facilitated by adsorption structures containing hydrophobic surfactant proteins. We examine biophysical evidence for and against the compression-driven squeeze-out mechanism and propose a new model for surfactant function. The proposed model is tested against existing physiological and pathophysiological evidence in neonatal and adult lungs, leading to ideas for biophysical research, that should be addressed to establish the physiological relevance of this new perspective on the function of the mighty thin film that surfactant provides.

Stability of multilamellar lipid tubules in excess water

In the lyotropic phase of lipids with excess water, multilamellar tubules (MLTs) grow from defects. A phenomenological model for the stability of MLTs is developed that is universal and independent of the underlying growth mechanisms of MLTs. The stability of MLTs implies that they are in hydrostatic equilibrium and stable as elastic objects that have compression and bending elasticity. The results show that even with solvent pressure differences of 0.1 atm, the density profile is not significantly altered, so suggesting the stability is due to the trapped solvent. The results are of sufficient value in relation to lamellar stability models and may have implications beyond the described MLT models, especially in other models of membrane systems.

Elucidating the Morphology of the Endoplasmic Reticulum: Puzzles and Perspectives

Artificial or synthetic organelles are a key challenge for bottom-up synthetic biology. So far, synthetic organelles have typically been based on spherical membrane compartments, used to spatially confine selected chemical reactions. In vivo, these compartments are often far from being spherical and can exhibit rather complex architectures. A particularly fascinating example is provided by the endoplasmic reticulum (ER), which extends throughout the whole cell by forming a continuous network of membrane nanotubes connected by three-way junctions. The nanotubes have a typical diameter of between 50 and 100 nm. In spite of much experimental progress, several fundamental aspects of the ER morphology remain elusive. A long-standing puzzle is the straight appearance of the tubules in the light microscope, which form irregular polygons with contact angles close to 120°. Another puzzling aspect is the nanoscopic shapes of the tubules and junctions, for which very different images have been obtained by electron microcopy and structured illumination microscopy. Furthermore, both the formation and maintenance of the reticular networks require GTP and GTP-hydrolyzing membrane proteins. In fact, the networks are destroyed by the fragmentation of nanotubes when the supply of GTP is interrupted. Here, it is argued that all of these puzzling observations are intimately related to each other and to the dimerization of two membrane proteins anchored to the same membrane. So far, the functional significance of this dimerization process remained elusive and, thus, seemed to waste a lot of GTP. However, this process can generate an effective membrane tension that stabilizes the irregular polygonal geometry of the reticular networks and prevents the fragmentation of their tubules, thereby maintaining the integrity of the ER. By incorporating the GTP-hydrolyzing membrane proteins into giant unilamellar vesicles, the effective membrane tension will become accessible to systematic experimental studies.

A Note on Vestigial Osmotic Pressure

Recent experiments have indicated that at least a part of the osmotic pressure across the giant unilamellar vesicle (GUV) membrane was balanced by the rapid formation of the monodisperse daughter vesicles inside the GUVs through an endocytosis-like process. Therefore, we investigated a possible osmotic role played by these daughter vesicles for the maintenance of osmotic regulation in the GUVs and, by extension, in living cells. We highlighted a mechanism whereby the daughter vesicles acted as osmotically active solutes (osmoticants), contributing an extra vestigial osmotic pressure component across the membrane of the parent vesicle, and we showed that the consequences were consistent with experimental observations. Our results highlight the significance of osmotic regulation in cellular processes, such as fission/fusion, endocytosis, and exocytosis.

Mesoscale simulations: An indispensable approach to understand biomembranes

Computer simulation techniques form a versatile tool, a computational microscope, for exploring biological processes. This tool has been particularly effective in exploring different features of biological membranes. In recent years, thanks to elegant multiscale simulation schemes, some fundamental limitations of investigations by distinct simulation techniques have been resolved. As a result, we are now capable of exploring processes spanning multiple scales beyond the capacity of any single technique. In this perspective, we argue that mesoscale simulations require more attention and must be further developed to fill evident gaps in a quest toward simulating and modeling living cell membranes.

Remodeling of Biomembranes and Vesicles by Adhesion of Condensate Droplets

Condensate droplets are formed in aqueous solutions of macromolecules that undergo phase separation into two liquid phases. A well-studied example are solutions of the two polymers PEG and dextran which have been used for a long time in biochemical analysis and biotechnology. More recently, phase separation has also been observed in living cells where it leads to membrane-less or droplet-like organelles. In the latter case, the condensate droplets are enriched in certain types of proteins. Generic features of condensate droplets can be studied in simple binary mixtures, using molecular dynamics simulations. In this review, I address the interactions of condensate droplets with biomimetic and biological membranes. When a condensate droplet adheres to such a membrane, the membrane forms a contact line with the droplet and acquires a very high curvature close to this line. The contact angles along the contact line can be observed via light microscopy, lead to a classification of the possible adhesion morphologies, and determine the affinity contrast between the two coexisting liquid phases and the membrane. The remodeling processes generated by condensate droplets include wetting transitions, formation of membrane nanotubes as well as complete engulfment and endocytosis of the droplets by the membranes.

Close, but not too close: a mesoscopic description of (a)symmetry and membrane shaping mechanisms

Biomembranes are fundamental to our understanding of the cell, the basic building block of all life. An intriguing aspect of membranes is their ability to assume a variety of shapes, which is crucial for cell function. Here, we review various membrane shaping mechanisms with special focus on the current understanding of how local curvature and local rigidity induced by membrane proteins leads to emerging forces and consequently large-scale membrane deformations. We also argue that describing the interaction of rigid proteins with membranes purely in terms of local membrane curvature is incomplete and that changes in the membrane rigidity moduli must also be considered.

Synthetic Membrane Shaper for Controlled Liposome Deformation

Shape defines the structure and function of cellular membranes. In cell division, the cell membrane deforms into a "dumbbell" shape, while organelles such as the autophagosome exhibit "stomatocyte" shapes. Bottom-up in vitro reconstitution of protein machineries that stabilize or resolve the membrane necks in such deformed liposome structures is of considerable interest to characterize their function. Here we develop a DNA-nanotechnology-based approach that we call the synthetic membrane shaper (SMS), where cholesterol-linked DNA structures attach to the liposome membrane to reproducibly generate high yields of stomatocytes and dumbbells. In silico simulations confirm the shape-stabilizing role of the SMS. We show that the SMS is fully compatible with protein reconstitution by assembling bacterial divisome proteins (DynaminA, FtsZ:ZipA) at the catenoidal neck of these membrane structures. The SMS approach provides a general tool for studying protein binding to complex membrane geometries that will greatly benefit synthetic cell research.

Membrane Manipulation of Giant Unilamellar Polymer Vesicles with a Temperature-Responsive Polymer

Understanding the complex behavior and dynamics of cellular membranes is integral to gain insight into cellular division and fusion processes. Bottom-up synthetic cells are as a platform for replicating and probing cellular behavior. Giant polymer vesicles are more robust than liposomal counterparts, as well as having a broad range of chemical functionalities. However, the stability of the membrane can prohibit dynamic processes such as membrane phase separation and division. Here, we present a method for manipulating the membrane of giant polymersomes using a temperature responsive polymer. Upon elevation of temperature deformation and phase separation of the membrane was observed. Upon cooling, the membrane relaxed and became homogeneous again, with infrequent division of the synthetic cells.

Non-equilibrium shapes and dynamics of active vesicles

Active vesicles, constructed through the confinement of self-propelled particles (SPPs) inside a lipid membrane shell, exhibit a large variety of non-equilibrium shapes, ranging from the formation of local tethers and dendritic conformations, to prolate and bola-like structures. To better understand the behavior of active vesicles, we perform simulations of membranes modelled as dynamically triangulated surfaces enclosing active Brownian particles. A systematic analysis of membrane deformations and SPP clustering, as a function of SPP activity and volume fraction inside the vesicle is carried out. Distributions of membrane local curvature, and the clustering and mobility of SPPs obtained from simulations of active vesicles are analysed. There exists a feedback mechanism between the enhancement of membrane curvature, the formation of clusters of active particles, and local or global changes in vesicle shape. The emergence of active tension due to the activity of SPPs can well be captured by the Young-Laplace equation. Furthermore, a simple numerical method for tether detection is presented and used to determine correlations between the number of tethers, their length, and local curvature. We also provide several geometrical arguments to explain different tether characteristics for various conditions. These results contribute to the future development of steerable active vesicles or soft micro-robots whose behaviour can be controlled and used for potential applications.